This invention relates generally to a monolithic concrete manhole base and, in particular, to an improved technique for manufacturing a manhole base in an upright position.
A poured concrete manhole base usually consists of a horizontally aligned pedestal that is adapted to support a vertical riser thereon. Generally, the riser is a hollow cylindrical member that may be used as a wet well or junction chamber for containing fluids. It is therefore important that the manhole possesses high strength and be leakproof to insure that the fluids will not escape or ground water enter, and thus degrade the system. By forming both the pedestal and the riser from a single piece of concrete, relatively high strength can be achieved and to a large extent the watertight integrity of the structure preserved. However, casting a one piece section of this type has proven to be rather difficult.
In order to overcome some of the problems associated with manufacture of a truly monolithic base, the base is sometimes cast in an inverted position. The mold contains a pallet upon which is seated a cylindrical closed top inner core and a cylindrical outer jacket in coaxial alignment to provide a space therebetween. The sidewall of the jacket is higher than that of the core so that concrete can be poured over the top of the core to create the pedestal. Upon curing sufficiently, until the concrete is hard and strong enough to be handled, the entire assembly is turned right side up and placed down on its base. The pallet, core and jacket are stripped from the concrete section and the concrete product is removed to storage. As can be seen, the component parts of the mold must remain assembled for a relatively long period of time to permit the concrete to cure, before the casting can be turned over without disturbing the concrete structure. It is therefore necessary to employ a large number of mold parts if any volume of production is to be maintained. This added equipment increases the cost of production and correspondingly raises the cost of the product.
By manufacturing the manhole base section in a normal upright position and using a semi-dry concrete mix, the core and the jacket may be quickly stripped from the product and reused in the manufacturing process. In U.S. Pat. No. 3,860,214, a method of manufacturing base sections in an upright position is disclosed. In this particular arrangement, the core is supported on the pallet in a raised position upon three legs. In assembly, the legs are locked to the pallet by a rather complex locking mechanism and the jacket then placed around the core. After placing the concrete, the core and the jacket are quickly stripped away without disturbing the product and thus can be rapidly turned around to once again form another base.
It must be noted, however, that the legs appended to the core produce three relatively large holes in the pedestal of the base section when the core is removed. These holes must be filled or somehow patched with concrete to complete the structure. The patched section thus represents a weak section in the base that can cause cracking which will eventually weaken the structure and/or permit contained fluids to leak therefrom thus disturbing the fluid handling capability of the system. It should be further noted that unlocking the legs from the pallet is difficult and can sometimes result in a time-consuming and relatively frustrating task. The pallets, because of the locking feature associated therewith, must be specially prepared and thus represent a relatively costly item which increases the cost of the product, particularly where large numbers are required.